Chemical looping processes are designed to transfer energy stored in carbonaceous fuels to metal oxide particles, which then release that energy in a separate reaction. Interest in chemical looping combustion processes for power generation has grown over the past few decades because the resulting carbon dioxide (CO2) can be sequestered without a need for an air separation unit (ASU).
Chemical looping processes employing a wide variety of system designs and flow regimes, e.g., moving bed, fluidized bed, and bubbling bed, have been designed to convert methane (CH4) to CO2.
A moving packed bed reactor in which a solid fuel such as coal is introduced at a midpoint of the moving packed bed reactor to set up a countercurrent flow pattern (upwardly flowing gas, downwardly flowing particles), which permits essentially complete conversion of the carbonaceous fuel into readily sequestered CO2 and water (H2O), is described in U.S. Pat. No. 7,767,191 and U.S. Pat. Publ. No. 2009/0000194. A metal oxide such as iron (III) oxide (Fe2O3) oxidizes coal, a reaction that reduces the metal oxide to a lower oxidation state (e.g., FeO) or even free metal (Fe). Reduced iron-containing particles are oxidized in a separate reactor (employing a packed or fluidized bed reactor), which fully regenerates the original metal oxide and produces significant amounts of heat that can be used to produce steam for power.
Researchers at Western Kentucky University have developed a chemical looping process that employs a combination of moving bed and bubbling fluidized bed (BFB) to fully oxidize a solid fuel like coal.
Chemical looping gasification methods are designed to produce hydrogen gas (H2) in addition to CO2. For example, researchers at the Instituto de Carboquímica have described a gasification process in which nickel oxide particles in a fluidized bed are employed to convert CH4 and H2O to syngas, a mixture of H2 and carbon monoxide (CO), as a primary product. The fuel stream includes H2O because production of H2 is the targeted output product of the equipment and process.
A chemical looping process that can convert a carbonaceous fuel, particularly a solid such as coal, to a gaseous output where CO2 is not the primary C-containing product and does not require the inclusion of an ASU remains of significant commercial interest.